To prevent the scp command from overwriting an existing file when transferring files between Unix or Linux systems, you can use the -n option. The -n option causes scp to perform a "dry run" of the copy operation, displaying the commands that would be executed but not actually transferring any files.
For example, to copy the file source.txt from the local system to the remote system, but prevent scp from overwriting an existing file with the same name, you can use the following command:
scp -n source.txt user@remote:/path/to/destination
This will display the scp command that would be used to copy source.txt to the remote system, but will not actually transfer the file. If there is already a file with the same name at the destination, scp will display an error message indicating that the file already exists, and the copy operation will not be performed.
It is important to note that the -n option does not prevent scp from overwriting existing files by default. If you want to prevent scp from overwriting existing files, you will need to specify the -n option along with any other options that you want to use.
For more information about the scp command and its options, you can consult the documentation for your specific Unix or Linux system.